A known measuring device for measuring the flatness (thickness variations) of the front and back sides of a thin disk (measuring object) having a large diameter such as a 300 mm diameter silicon wafer includes a support unit for supporting a measuring object in parallel with a prescribed plane such as a vertical plane, a first and second linear guide rail extending in parallel with the prescribed plane on either side of the measuring object, a first and second slider individually slidably supported by the first and second guide rails, respectively, a first measuring means (displacement meter) mounted on the first slider for measuring a distance to the front side (front face) of the measuring object, and a second measuring means for measuring a distance to the back side (rear face) of the measuring object. See Patent Document 1, for instance.
In such a measuring device, an individual air slide mechanism is required to be provided on each side of the support unit supporting the measuring object (extending vertically). By placing each guide rail at a height corresponding to the height at which the displacement meter scans the measuring object such as a silicon wafer, the distance between the guide rail and the displacement meter can be reduced as compared to the case where the guide rail is provided in a lower part of the support unit.
Thereby, the roll error caused by the angular displacement of the slider around the axial center line of the guide rail can be minimized as this error increases in proportion to the vertical distance between the guide rail and the displacement meter. By reducing the roll error, the flatness of both the front and back sides can be individually measured at a high precision.